Mesoporous structures refer to high-surface area porous oxides, such as silicon oxides, having an average pore size of not greater than about 100 nanometers as calculated using the nitrogen adsorption/desorption isotherm, as disclosed in Stucky et al., US Patent Publication 2009/0047329 incorporated herein by reference in its entirety. Some mesoporous oxide structures can be prepared in the form of mesocellular foams. Mesoporous silicon oxide based structures are believed to be useful in a variety of applications, such as thermal insulation, treatment of bleeding wounds, catalysis, molecular separations, fuel cells, adsorbents, patterned-device development, optoelectronic devices and in biological sensors, among others. These mesoporous structures provide relatively low cost, ease of handling and high resistance to photo-induced corrosion.
Mesoporous structures are generally prepared by exposing a source of a metal or metalloid oxide, e.g. silicon oxide, for instance tetraethylorthosilicate, to cross-linking conditions with a micro-emulsion or emulsion of surfactants, and optionally micelle swelling agents, in water. The metal or metalloid oxide, silicon oxide, crosslinks to form on the surface of the micelles of the surfactant, and optionally micelle swelling agent, to form the basic mesoporous structure. The size of the pores is related to the size of the micelles formed. The size of the micelles can be adjusted by swelling with one or more micelle swelling organic solvents. The reaction medium containing the mesoporous structures is exposed to elevated temperatures so as to further adjust the pore structure and properties. The mesoporous structures are separated from the aqueous reaction medium and exposed to temperatures at which any organic materials are removed by volatilization and/or burning them out. The structure of the mesoporous materials may be altered by heating to temperatures at which they undergo calcination, up to 500° C.
Early mesoporous structures were reported to be crystalline as evidenced by the presence of patterns in XRD profiles (2θ=0-10) obtained by X-ray powder diffraction and exhibit mesopores of the size of about 1.0 to about 100 nanometers. See Kresge et al. U.S. Pat. No. 5,098,684; Beck et al. U.S. Pat. No. 5,304,363 U.S. Pat. No. 5,304,363; and Kresge et al. U.S. Pat. No. 5,266,541, incorporated herein by reference in their entirety. Such mesoporous silicon oxide based structures are disclosed as being crystalline in nature, brittle and having thin pore walls. Pinnavia et al. U.S. Pat. No. 6,641,657: and U.S. Pat. No. 6,506,485, incorporated herein by reference in their entirety, address this issue by preparing amorphous highly crosslinked silicon oxide mesoporous structures. Such mesoporous structures are disclosed to have a connectivity Q4/(Q3+Q2) ratio of 2.5 to 8.0, wherein Q4 is the number of silicon oxide units having four bonds to other silicon oxide units, Q3 is number of silicon oxide units having three bonds to other silicon oxide units and Q2 is number of silicon oxide units having two bonds to other silicon oxide units. Such mesoporous structures can be relatively dense, exhibit relatively low pore volumes and have few silanol groups. For certain uses, such as in insulation foams, high pore volume materials are desired. In other uses high silanol concentrations are desirable, for instance where it is desirable to bond functional compounds into the mesoporous structures. See also Chmelka et al., US 2006/0118493; and Stucky US 2009/0047329 incorporated herein by reference in their entirety.
Processes for preparing known mesoporous silicon oxide based structures present challenges. Chmelka et al. and Stucky et al. disclose the use of tetraalkyl orthosilicates, such as tetraethyl orthosilicate, as a source of silicon oxide. Tetraalkyl orthosilicates are relatively costly, which limit some of the applications of mesoporous structures prepared therefrom. In addition, the use of tetraalkyl orthosilicates results in the generation of alkanol(s) byproducts, the presence of which can affect micelle structures and introduce variability in the resulting mesoporous structure, for instance a broader pore size distribution. Pinnavaia et al. U.S. Pat. No. 6,641,657; and U.S. Pat. No. 6,506,485 disclose the use of water soluble silicates, ionic silicates, as the source of silicon oxide. The ionic silicates leave residual ions, such as alkali metal ions, in the resulting product and in the aqueous mixture left behind after recovery of the crosslinked silicon oxide highly mesoporous precursor. The presence of the ions in the ultimate mesoporous structures can present problems for certain uses. The aqueous mixtures can contain ions in the aqueous mixture left behind after recovery of the crosslinked silicon oxide mesoporous structures and may contain surfactants and organic materials after preparation of the precursors. This can present challenges with respect to disposal of the aqueous mixtures. Many disclosed processes for the preparation of mesoporous silicon oxide structures utilize calcination steps which require exposure of the precursors to high temperatures for extended time periods, which can be costly and energy intensive.
What are needed are mesoporous silicon oxide based structures which are amorphous, exhibit high pore volumes and low densities, contain relatively high levels of silanol groups along the backbone of the silicon oxide chains and contain low residual ion levels. What are needed are processes for preparing such mesoporous structures that utilize cost effective sources of silicon oxide, which do not require the presence of metal ions, do not require the use of a calcination step and which allow for the recovery and reuse of organic materials used in preparing the highly porous mesoporous siliceous structures.